The present invention relates generally to the field of EIT, and in particular to a new and useful electrical impedance imaging apparatus and a method for calibrating the electrical impedance imaging apparatus.
Electrical impedance tomography is an imaging modality which displays the spatial distribution of the complex conductivity distribution inside a body. An excitation is applied to electrodes on the body surface, resulting in an electromagnetic (EM) field appearing within the volume. If the excitation consists of one or more currents, the voltages that appear at some or all of the electrodes are measured. If the excitation consists of voltages, then the currents at the electrodes are measured. The inverse problem describing the current to voltage relationship is then solved to determine the complex conductivity distribution that must have been present to produce the measured data set. The ill-posedness of this inverse problem manifests itself through the small changes in surface current or voltage that sometimes result from large changes in the interior impedance distribution. High precision electronics are needed to apply the excitations and measure the data that correspond to these changes.
For an electrical impedance tomograph with finite measurement precision, distinguishability is defined as the ability to detect an inhomogeneity in a homogeneous background, and is maximized for all conductivities and geometries when currents are applied to the surface electrodes and the resulting voltages are measured. Furthermore, distinguishability is maximized when multiple, independent current sources are used to apply spatial patterns of currents to the electrodes. Applying current patterns whose eigenvalues match the natural response (modes) of the body being interrogated maximizes the signal to noise ratio (SNR) of the resulting data set and therefore minimizes the amount of regularization necessary for the subsequent reconstruction.
Taken together, these two observations suggest that multiple, high-precision current sources are required to maximize the distinguishability and SNR of an impedance tomography data set. While such current sources have been developed for this application, they tend to have limited bandwidth, apply only sinusoidal excitations, and require a large number of high precision components. (See [1] R. D. Cook, G. J. Saulnier, D. G. Gisser, J. C. Goble, J. C. Newell, and D. Isaacson. ACT3: A high-speed, high-precision electrical impedance tomograph. IEEE Transactions on Biomedical Engineering, vol. 41 (8): 713-722, August 1994; Also see [2] A. S. Ross, G. J. Saulnier, J. C. Newell, and D. Isaacson. Current source design for electrical impedance tomography. Physiological Measurement, vol. 24(2):509-516, May 2003.) The result is an electronics package with a large system footprint, and high component, power, and cooling costs.
In contrast, precision voltage sources are generally easier and less costly to implement. However, as mentioned above, applying voltages and measuring currents produces a less optimal EIT system. In an attempt to gain the hardware simplicity of a voltage source along with the optimality of applied currents, algorithms have been developed for utilizing multiple voltage sources to apply a desired current pattern (See [3] M. H. Choi, D. Isaacson, G. J. Saulnier, and J. C. Newell. An iterative approach for applying multiple currents to a body using voltage sources in electrical impedance tomography. In Proceedings of the 25th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, volume 4, pages 3114-3117, September 2003.) This algorithm takes into account the interaction between the sources which results in the current at any given electrode being a function of the voltages on all the electrodes.
When using a voltage source in EIT, it is necessary to know both the applied voltage and the resulting current with high precision. If it is desired to have the applied voltage remain unchanged for a wide range of load impedances, then it is necessary to have a voltage source with low output impedance. In the case where voltages are being applied to produce specific currents, the applied voltage will be adjusted, generally in an iterative way, to compensate for changes in the load impedance. Consequently, low output impedance less important than having the ability to precisely measure the actual applied voltage and current.